Revised corn 2 class20

Editor's Notes

• #5: 1 st ECGs were recorded in late 1800s Einthoven ’s ECG machine
• #6: Basically, it is a recording of the electrical activity of the heart ECG leads detect the electrical current generated by the depolarization and repolarization of the heart. From cardioweb.co.uk
• #8: The electrical current caused by the depolarization and repolarization of the ATRIA of VENTRICLES is called cardiac vector Visualized graphically as an ARROW
• #9: The length of the arrow represents the MAGNITUDE or SIZE of the electric current The position (or orientation) of the arrow indicates the direction of flow of the electric current POLARITY tells us which is positive and negative. The tip is + pole , and the tail is – pole Huszr, 2002, p. 255
• #10: Each wave has a vector: e.g. P wave, T wave, QRS. We are going to be analyzing QRS vector
• #11: Garcia & Holtz, 2001
• #12: Each lead is obtained using a Positive and Negative electrode Positive electrode can be attached to the extremities: R arm, L arm or L leg. Negative electrode (can be attached to an extremity), but with ECG machines, it is created electronicaly! negative electrode is an ‘indifferent’ zero reference point  Central Terminal (CT). Hypothetically, the CT is located in the electrical center of the heart (left of the IV septum & below the AV junction). With CT, do NOT need a separate external negative electrode(Huszar, 2002). Ground electrode – attached to any location on the body – to provide a path of least resistance for electrical interference in the body
• #13: Each lead is obtained using a positive and negative electrode Positive electrode is attached to the extremities (R arm, L arm, L leg) to obtain the six limb leads (standard and augmented) Central terminal – negative electrode – created electronicaly within the circuitry of the ECG machine: hypothetically located in the heart – where the electrical center of the heart is located (IV septum, below AV junction)
• #14: BIPOLAR leads: I, II, and III  detect electrical activity between 2 electrodes: the positive and negative that are attached to the extremities UNIPOLAR leads: detect the electrical activity generated by the heart by using a positive electrode (attached to 1 of 3 extremities (augmented leads), or chest leads and the Central Terminal. (Huszar, 2002). 
• #15: Limb leads usually on extremity or shoulder – but should be at least 10cm away from heart.
• #19: AUGMENTED LEADS: a = augmented, V = voltage, R, L and F indicate where the positive electrode is placed: R arm, L arm and L leg Augmented leads: called augmented, when it was discovered that by eliminating a negative electrode resulted in the amplitude of the recording being augmented by 50% Augmented leads: positive electrode on each shoulder and left leg, negative electrode is at heart
• #21: Each lead of the 12 lead ECG measures the difference in electrical potential between the postive and negative electrodes or central terminal (or poles). Therefore, each lead has a positive and negative pole. A hypothetical line joining the poles of a lead is known as the axis of the lead (or lead axis). The location of the positive and negative poles in a lead determines the orientation (OR direction) of the axis of the lead. THUS, a lead axis has direction and polarity,.
• #22: Huszar, 2002, p. 246 Lead I Axis and its perpendicular (intersects at Right angle or 90  angle)
• #23: Looking at heart
• #24: Huszar, 2002 p. 246 Frontal plane: Looking at patient Horizontal plane: 6 chest leads measure activity of heart at a Right angle to the frontal plane Cutting through patient
• #25: Knowing Trigonometry Einthoven ’s triangle is formed by the axes of the 3 bipolar limb leads: I, II, and III: Because the + electrodes of the 3 standard limnbe leads are electrically about the same distance from the zero reference point in the heart, and equilateral  can be depicted in the body ’s frontal plane using the 3 lead axes Einthoven ’s Law: the sum of the electrical current recorded in lead II is equal to the sum of the electrical current recorded in leads I and III  I + III = II OR I + III + (-II) = 0 (cancel each other out,  this combination creates a zero potential – used as a reference point for the unipolar leads
• #26: Vector mathematics/physics
• #27: Huszar, 2002, p. 248 Remember, a circle is 360  Triaxial reference figure for standard limb leads depicts Leads I, II, and III and their axes Leads are spaced 60  apart
• #28: Remember, the augmented leads: aVR, aVL, and aVF are obtrained using a + electrode attached to 1 of 3 extremities, and the CT (electrical center of the heart). Augmented leads measure the electrical activity between the positive electrodes and the negative or CT Triaxial reference figure for augmented leads: similar to standard limb leads: lead axes are 60 ° apart, BUT oriented around the zero refernce point at slightly different angles! Huszar, 2002, p. 249
• #29: Why do we need to know this?? Will be using the hexaxial reference figure to determine the Axis!
• #30: Chest leads are unipolar leads: Detect heart ’s electrical current using a positive electrode attached to various locations on the chest wall, and the central terminal.
• #31: Huszar, 2002: V1 and V2: 4 th intercostal space (right and left of sternum) V3-6: 5 th intercostal space
• #33: Huszar, 2002, p. 250 Remember, chest leads: positive electrodes, and negative electrode is the CT V1 and 2 (septal leads): lie over the RV V3 and 4 (anterior leads): lie over IV septum and part of the LV V 5 and 6 (Lateral leads): lie over rest of LV Lead axis for each chest lead is drawn from the CT (- pole) to the specific chest electrode (+ pole) Precordial reference figure: used to plot heart ’s electrical activity in the Horizontal plane
• #34: Localizing an infract
• #35: Huszar, p. 251 12 leads provide 12 different views of the electrical acticvity of the heart. Each view looks from the outside of the chest toward the zero reference point within the chest ** Need to know facing leads in determining the location of an AMI** Facing leads: Leads I, aVL, V5 and V6: view lateral surface of the heart Leads II, III, and aVF : view inferior (diaphragmatic) surface of the heart V1 – V4: look at anterior surface of the heart NO leads face the posterior surface of the heart R sided chest lead V4R – looks at RV (not a facing lead, it faces the interior, endocardial surface of the ventricles)
• #36: Answer is B
• #37: Answer is: B
• #38: Answer if C. INferior
• #39: Huszar, 2002, p. 255 Figure 12-1 This slide shows the sequence of electrical currents produced by depolarization of ventricles during one cardiac cycle  represents depolarization of the IV septum and is directed from Left to R  shows series of vectors produced by depolarization of the ventricles (R and L), in the apical region of the heart near the septum  as depolarization continues – through the walls of the RV and LV  depolarization ends with lateral & posterior aspect of the LV near its base NOTE: - Vectors arising in the RV are directed mostly to the Right when viewed in the frontal plane (from patient ’s right)
• #40: QRS points to the left and downward – reflecting dominance of LV over the RV T axis may be determined in certain conditions: e.g. myocardial ischemia and acute MI – to help localize affected area
• #41: Electrical current flowing parallel to the axis of a lead (hypothetical line joining the – and + poles), will produce either a positive or negative deflection depending on which way the current is flowing. Towards the positive pole: positive deflection Towards negative pole: negative ** the greater the magnitude (size) of the electrical current, the larger the deflection, and vice versa (the smaller the size, the smaller the deflection)
• #42: Huszar, p. 257
• #43: Back to the hexaxial reference figure Primary purpose of hexaxial reference figure is : to aid in the determination of the direction of the QRS axis in the frontal plane
• #44: p. 259, Huszar Hexaxial figure  into 4 quadrants by the bisection of lead axes I and aVF Quadrant I  0 to -90  Quadrant II  0 to +90  III  +90 to +/- 180  IV  -90 to +/- 180
• #45: Normal QRS axis lies between –30 and +90  A change or SHIFT in the QRS axis from normal to one between –30 and -90  is considered a LAD (left axis deviation) Shift to the Right = RAD (between +90 and +/- 180) No man ’s land (indeterminate axis): rarely falls in this quadrant
• #46: Most common causes of abnormal axis shift include:
• #47: Systemic hypertension, aortic stenosis, ischemic heart disease  lead to LV enlargement and hypertrophy
• #48: See page 262 Disorders that cause RV enlargement and hypertophy such as COPD, Pulmonary embolus – lead to RAD **In emergency situations: Determination of QRS axis may be useful in acute MI - to determine if anterior or posterior fascicular blocks are present, or in tachycardia with wide QRS to diff between VT and SVT